Magnetic Random Access Memory and Fabricating Method Thereof

The present invention relates to a magnetic random access memory (MRAM) and a fabricating method thereof, and more particularly, to an MRAM device integrated with circuitry which is formed under a complementary metal oxide semiconductor (CMOS) process.

A magnetic memory element has a structure which includes ferromagnetic layers separated by a non-magnetic layer. Information is stored as directions of magnetization vectors in magnetic layers. Magnetic vectors in one magnetic layer, for instance, are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer is free to switch between the same and opposite directions as information which are called "Parallel" and "Antiparallel" states, respectively. In response to Parallel and Antiparallel states, the magnetic memory element represents two different resistances. The resistance indicates minimum and maximum values when the magnetization vectors of two magnetic layers point in substantially the same and opposite directions, respectively. Accordingly, a detection of changes in resistance allows an MRAM device to provide information stored in the magnetic memory element.

An MRAM device integrates magnetic memory elements and other circuits, for example, a control circuit for magnetic memory elements, comparators for detecting states in a magnetic memory element, input/output circuits, etc. These circuits are fabricated in the process of CMOS technology in order to lower the power consumption of the MRAM device. The CMOS process requires high temperature steps which exceeds 300.degree. C. for depositing dielectric and metal layers and annealing implants, for example.

Magnetic layers employ ferromagnetic material such as CoFe and NiFeCo which requires processing below 300.degree. C. in order to prevent intermixing of magnetic materials caused by high temperatures. Accordingly, magnetic memory elements need to be fabricated at a different stage after CMOS processing.

Magnetic memory elements contain components that are easily oxidized and also sensitive to corrosion. To protect magnetic memory elements from degradation and keep the performance and reliability of the MRAM device, a passivation layer is formed over magnetic memory elements.

In addition, a magnetic memory element includes very thin layers, some of them are tens of angstroms thick. The performance of the magnetic memory element is sensitive to the surface conditions on which magnetic layers are deposited. Accordingly, it is necessary to make a flat surface to prevent the characteristics of an MRAM device from degrading.

Metal lines are employed to produce magnetic fields for writing and/or reading states in a magnetic memory element. Less amount of current is desired to minimize power consumption.

Accordingly, it is a purpose of the present invention to provide an improved MRAM device which prevents a magnetic memory element from thermal degradation while fabricating the device.

It is another purpose of the present invention to provide an improved MRAM device which prevents a magnetic memory element from oxidation and corrosion.

It is a further purpose of the present invention to provide an improved MRAM device which reduces power consumption of the device.

It is a still further purpose of the present invention to provide a method of integrating an improved MRAM device into a CMOS process.

These needs and others are substantially met through provision of a magnetoresistive random access memory (MRAM) which includes magnetic memory elements on circuitry for controlling operations of magnetic memory elements. First, the circuitry is formed on a substrate under the CMOS process which requires a heat treatment of 300.degree. C. or more. While fabricating the circuitry, conductive lines are also formed, which are used to create magnetic fields for writing and/or reading states in the magnetic memory element. The metal lines are enclosed by high permeability material such as a permalloy layer which facilitates magnetic fields to concentrate on the magnetic memory element. After completion of the circuitry, a surface of a layer including the circuitry is polished by the chemical mechanical polishing (CMP) process which produces a flat surface on the layer including the circuitry, then the magnetic memory element is formed thereon. The flat surface prevents the characteristics of the magnetic memory element from degrading. Fabrication of the magnetic memory element after the CMOS process improves the performance and reliability of the magnetic memory element and avoids thermal degradation of the magnetic memory element.

FIGS. 1-13 show cross-sectional views of sequential steps in the formation of an MRAM device; and

FIGS. 14-17 show cross-sectional views of sequential steps in the formation of another embodiment of an MRAM device.

FIGS. 1-13 illustrate cross-sectional views of sequential steps for fabrication of an MRAM device which includes transistors for switching magnetic memory elements in reading operations.

Referring specifically to FIG. 1, a cross-sectional view of a partially fabricated magnetoresistive random access memory (MRAM) device 10 is illustrated, wherein device 10 includes a P-type silicon substrate 11. Device 10 has circuitry, for instance, NMOS switching transistors 12a and 12b which are fabricated by the well known CMOS process. Other circuit elements, for example, input/output circuit, data/address decoder, and comparators, may be contained in the MRAM device, however they are omitted from the drawings for simplicity.

First of all, substrate 11 is provided to pattern windows for N+ regions 13a, 13b and 13c and implant the source/drain regions 13a, 13b and 13c. Then isolation regions 14a and 14b are formed for separation. Next, poly-Si layers 15a and 15b are deposited on substrate 11 for forming gate regions. Metal layers 16a and 16b are deposited on N+ region 13a and 13b for source electrodes while metal layer 16c is deposited on N+ region 13c for a drain electrode. Further, metal layers 17a and 17b for gate electrodes are deposited on poly-Si layers 151 and 15b, respectively. A conductor line 18 is formed on metal layer 16c, which provides a sense current to magnetic memory elements through transistors 12a and 12b. A magnetic memory element will be explained hereinafter. Plug conductors 19a and 19b, which work for conducting a sense current to magnetic memory elements, are formed on and interconnected to metal layers 16a and 16b, respectively. All circuit elements of an MRAM device, except magnetic memory elements, digit lines and word lines, are integrated on substrate 11 before dielectric material 20 is filled. Then, A surface of device 10 is polished by the CMP (Chemical Mechanical Polishing) process which includes the top surface of dielectric layer 20 is flat.

After partially fabricated MRAM device 10 has been completed, magnetic memory elements are formed on device 10 along with digit lines and word lines. As shown in FIG. 2, an etch stop layer 21, which employs material such as AlN, AlO and SiN, is deposited on the surface of device 10. Instead of etch stop layer 21, other technique such as endpoint etches may be used. A silicon dioxide layer 22 is deposited with a thickness of 4,000-6,000 .ANG. on etch stop layer 21.

In the next step, a mask layer is deposited on silicon dioxide layer 22 and is patterned and defined as an etching mask using a standard lithography technique. As shown in FIG. 2, silicon dioxide 20 is etched away to etch stop layer 21 that makes trenches 23a-23d in silicon dioxide layer 22, and then the exposed etch stop layer is removed from trenches 23a-23d.

Referring to FIG. 3, a thin field focusing layer 24 having a high permeability such as nickel-iron is deposited overlying trenches 23a-23d and a silicon dioxide dielectric layer 25. High permeability layer 24 is 100-500 .ANG. thick. In order to improve adhesion of field focusing layer 24 and to provide a barrier for Ni or Fe diffusion into dielectric layer of Ta or TaN or other such materials could be added between field focusing layer 24 and dielectric layer 25. A conductor metal layer 26 is then deposited on field focusing layer 24. As a conductor metal, aluminum, aluminum alloys, copper, and copper alloys are employed. In order to improve adhesion of field focusing layer 24 and to provide a barrier for Ni or Fe diffusion into the conductor and/or dielectric a layer of Ta or TaN or such materials could be added between field focusing layer 24 and conductor layer 26. After depositing metal layer 26, the metal bulged out of trenches 23a-23d and the high permeability layer 24 on silicon dioxide layer 25 is removed from a top surface by the CMP process so that, as shown in FIG. 4, a partially fabricated MRAM device 27 having a flat top surface 28 is produced.

Partially fabricated MRAM device 27 includes torque or digit lines 29 and 30 on which magnetic memory elements are formed. Digit lines 29 and 30 carry a current to generate a magnetic field which causes magnetic memory elements to store states. Digit lines 29 and 30 are enclosed by high permeability layers 31 and 32 excluding a portion on the top surface 28. Layer 31, for example, shields the magnetic field generated by current flowing in digit line 29 from magnetic flux leakage, and facilitates the magnetic field to focus on a magnetic memory element placed on digit line 29 through top surface 28 not covered by layer 31.

Referring to FIG. 5, a dielectric layer 33 is deposited over digit lines 29 and 30 and dielectric layer 25, and a conductor layer 34 is deposited over dielectric layer 33. Dielectric layer 33 is placed between digit lines 29 and 30 and conductor layer 34 to provide electrical isolation therebetween. Dielectric layer 33 is partially etched to make windows 35 and 36 on metal conductors 37 and 38 which are employed to electrically connect plug conductors 19a and 19b to conductor layer 34. After making windows 35 and 36, conductor layer 34 is deposited with a thickness of around 500 .ANG. over dielectric layer 33 and metal conductors 37 and 38. In order to form magnetic memory elements on conductor layer 34, a top surface of conductor layer 34 needs to be smooth and flat because magnetic memory elements have very thin films, thereby a good condition for a magnetic memory element is attained. Surface 39 is polished and formed by a planarizing process such as CMP.

Next, as shown in FIG. 6, layers 40-42 for magnetic memory elements are deposited by either plasma vapor deposition (PVD) or ion beam deposition (IBD) techniques on surface 39 of conductor layer 34. Bottom and top magnetic layers 40 and 42 utilize magnetic material such as CoFe and NiFeCo while layer 41 employs a non-magnetic material such as Al.sub.2 O.sub.3 or Cu. Bottom layer 40, for example, serves as a hard magnetic layer, magnetization in which is pinned or fixed, whereas magnetization directions in top magnetic layer 42 are free. Non-magnetic layer 41 is formed by the following methods. An aluminum film is deposited over bottom magnetic layer 40, then the aluminum film is oxidized by an oxidation source, such as RF oxygen plasma. As another method, aluminum is deposited together with oxide on layer 40, and then oxidation is carried out in oxygen ambient either heated or unheated. The layers in the magnetic memory element are very thin with magnetic layers varying from 20 to 200 .ANG. and non-magnetic layer 41 from 10 to 30.ANG..

Layers 40-42 are patterned and etched down to conductor layer 34 so that, as shown in FIG. 7, magnetic memory elements 43 and 44 are formed on conductor layer 34. Furthermore, a mask pattern is formed on conductor layer 34 and etched down to dielectric layer 33 to make conductor lines 45 and 46 which electrically couple between magnetic memory element 43 and transistor 12a through plug conductor 19a, and magnetic memory element 44 and transistors 12b through plug conductor 19b, respectively. Conductor lines 45 and 46 are isolated from digit lines 29 and 30 by dielectric layer 33, respectively.

Referring to FIGS. 8 and 9, an enlarged cross-sectional view of an MRAM device 47 and a partial cross-sectional view through digit line 29, magnetic memory element 43, and a bit line 48 indicated by arrows 9--9 in FIG. 8 are illustrated. It should be noted that although three bit lines 48 and 49 and three magnetic memory elements 43 and 50 are shown in FIG. 9 for simplicity, more bit lines and magnetic memory elements are formed for MRAM device 47.

Referring to FIG. 8 again, after the etching of layers 40-42 and conductor layer 34 has been completed in order to form magnetic memory elements 43 and 44 as shown in FIG. 7, dielectric layer 51 is deposited overlying magnetic memory elements 43 and 44. Then, dielectric layer 51 on magnetic memory elements 43 and 44 is etched to define holes to magnetic memory elements 43 and 44, and conductive material is deposited in the holes to form conductive lines 52 and 53. Next, an etch stop layer 54 is deposited on dielectric layer 51 and, further, a dielectric layer 55 in FIG. 9 is deposited on etch stop layer 54. Next, a mask is patterned on dielectric layer 55 to form trenches for bit lines 48 and 49.

According to the mask, as shown in FIG. 9, dielectric layer 55 is etched down to etch stop layer 54 to make the trenches for bit lines 48 and 49. Next, a permalloy layer 56 is deposited over dielectric layer 55 and in the trenches. The permalloy layer is etched by an anisotropic etching which leaves only a permalloy layer 56 on the sidewalls of the trenches and removes the permalloy layer on dielectric layer 55 and the bottom of the trenches. After forming permalloy layer 56, a layer 57, which is electrically conductive, is deposited on the surface of conductive lines 52 and 69, and a metal such as Al, W, and Cu is filled in the trench for forming bit lines 48 and 49. Next, unnecessary material on dielectric layer 55 is removed and the surface of dielectric layer 55 and bit lines 48 and 49 is polished to a flat surface. Finally, a permalloy layer 58 is deposited and patterned on dielectric layer 55 and bit lines 48 and 49. Permalloy layers 56 and 58 enclose bit line 48, by which a magnetic field generated by a bit current in bit line 48 is concentrated toward magnetic memory element 43 and shielded to protect information in other magnetic memory elements.

Referring to FIG. 8 again, reading and writing operations of the MRAM device are briefly discussed hereinafter. Assume that magnetic memory element 43 is selected, for example. In a reading operation, a turn-on signal is applied to gate electrode 17a to turn on transistor 12a which allows a sense current to flow from drain electrode 18 through metal layer 16a, plug conductor 19a, conductor line 45 and magnetic memory element 43, to bit line 48. The sense current produces a voltage drop over magnetic memory element 43 which determines states stored in magnetic memory element 43. That is, the voltage drop is provided to voltage comparators (not shown) which compare with reference voltages to give states stored in magnetic memory element 43.

In order to write states in magnetic memory element 43, for example, bit and digit currents, which are provided to bit line 48 and digit line 29, generate magnetic fields, respectively. The magnetic field generated by the digit current is superimposed on the magnetic field generated by the bit current so that the total magnetic field is applied to write states in magnetic memory element 43. Directions of combined magnetic fields determine magnetization directions in free magnetic layer 42, thereby magnetic memory element 43 memorizes states or information. Digit line 29 is enclosed by permalloy layer 31 while bit line 48 is enclosed by permalloy layer 56 and 58. Permalloy layers 31, 56 and 58 facilitate magnetic fields generated by digit and bit currents to focus on magnetic memory element 43. Accordingly, less current is required to write and sense states in the magnetic memory elements.

Another embodiment for forming a bit line is illustrated in FIGS. 10 and 11. FIG. 11 is a partial cross-sectional view through digit line 29, magnetic memory element 43, and a bit line 59 indicated by arrows 11--11 in FIG. 10. After the formation shown in FIG. 7 has been completed, as shown in FIG. 10, a dielectric layer 51 is deposited overlying magnetic memory elements 43 and 44 and the top surface of the dielectric layer 51 is polished. Then an etch stop layer 60 and a dielectric layer 61 on the surface of etch stop layer 60 are deposited sequentially. A mask is deposited and patterned on the dielectric layer 61 to form trenches for bit lines 59 and 62 in FIG. 11.

Next, referring to FIG. 11, dielectric layer 61 is etched until etch stop layer 60 according to the mask so that trenches are formed for bit lines 59 and 62. Then, a permalloy layer is deposited on dielectric layer 61 and in the trenches. The permalloy layer is patterned and etched by an anisotropic etching which leaves only a permalloy layer 63 on the sidewalls of the trenches. Further, the bottom of the trench is etched down until the top surface of magnetic memory elements 43 and 66 to form concave portions 64 and 65 which connect magnetic memory element 43 to bit line 60 and magnetic memory elements 66 to bit line 62, respectively. The bottom surface of the trench is deposited with a layer 67 such as Ta, TaN, or TiN, etc. which electrically connects between magnetic memory elements 43 and 66 and bit lines 59 and 62, respectively. A conductive material such as Cu is plated and filled in concave portions 63 and 64 and the trenches which form bit lines 59 and 62. The top surface of bit lines 59 and 62 and dielectric layer 59 are then polished and a permalloy layer 68 is deposited and patterned thereon. Bit line 59, for example, is enclosed by permalloy layers 63 and 68 which facilitate a magnetic field generated by a bit current in bit line 59 to focus on magnetic memory elements 43. The reading and writing operations are carried out by the same steps as mentioned earlier.

FIGS. 12 and 13 further illustrate another embodiment for forming bit lines 70 and 71. FIG. 13 is a partial cross-sectional view through digit line 29, magnetic memory element 43, and a bit line 70 indicated by arrows 13--13 in FIG. 12. After magnetic memory elements 43 and 44 are formed as shown in FIG. 7, a dielectric layer 72 is deposited overlying elements 43 and 44, and patterned and etched down to a top surface of elements 43 and 44 for making trenches 73 and 74. Next, metal material such as Al and Cu is filled in trenches 73 and 74 to form bit lines 70 and 71. The dielectric layer overlying bit line 73 and 74 is removed and a field focusing layer 75 such as NiFe is deposited on bit line 73 and 74 and dielectric layer 72. Then a dielectric layer 76 is deposited on field focusing layer 75.

FIGS. 14-17 illustrate cross-sectional views of sequential steps in the formation of another embodiment of an MRAM device which includes diodes for switching a magnetic memory element to read information in the magnetic memory element. A cross-sectional view in FIG. 14 shows a partially fabricated MRAM device 80 which includes a substrate 81 having CMOS circuitry (not shown), and digit lines 82 and 83 enclosed by permalloy layers 84 and 85, respectively. This structure corresponds to digit lines 29 and 30 and permalloy layers 31 and 32 in FIG. 4. Accordingly, the same steps are carried out to produce the formation of digit lines 82 and 83 and permalloy layer 84 and 85. After depositing dielectric layer 86 overlying digit lines 82 and 83, the top surface of dielectric layer 86 is polished by the CMP process.

Referring to FIG. 15, several layers for diodes and magnetic memory elements are deposited on polished dielectric layer 86. First of all, a couple of diode layers 87 and 88 are formed for a diode, for example, pn junction diodes or Schottky barrier diode. Next, magnetic layers 89, non-magnetic layer 90 and magnetic layer 91 are sequentially deposited on layer 88, which forms the same layers 89-91 as layers 40-41 in FIG. 6. A mask (not shown) is then deposited and patterned on a top surface of layer 91. According to the mask, layers 89-91 and diode layers 87 and 88 are etched down to dielectric layer 86. As shown in FIG. 14, the etching forms magnetic memory element 92 on diode 93, and magnetic memory element 94 on diode 95.

Referring to FIG. 17, a final formation of MRAM device employing a diode connection is illustrated. After the etching of layers 89-91 and diode layers 87 and 88, dielectric layer 96 is deposited overlying magnetic memory elements 92 and 94 and diodes 93 and 95. A mask is then deposited on dielectric layer 96 to selectively open windows 97 and 98 which are used to communicate top surfaces of magnetic memory elements 92 and 94 to a bit line 99. Bit line 99 such as Al and Cu is deposited on dielectric layer 96 and magnetic memory elements 92 and 94. Further, a permalloy layer 100 is deposited and patterned on bit line 99 to shield a magnetic field generated by a bit current in bit line 99 and focus it on magnetic memory elements 92 and 94.

In a reading operation to magnetic memory element 92, for example, a sense current is applied to magnetic memory element 92 through diode 93 from digit line 82. A voltage drop over magnetic memory element 92 is produced according to the states, which is sensed and compared to a reference voltage by a comparator (not shown) for the two possible states of magnetic memory element.

In a write operation, a bit current and a digit current is provided to bit line 97 and digit line 82, respectively. A combination magnetic field produced by bit and digit currents is applied to magnetic memory element 92. Magnetization directions of the combination field determines states to be stored in magnetic memory element 92.

In FIG. 17, permalloy layer 100 covers only the top surface of bit line 99. It should be appreciated that bit line 99 and permalloy layer 100 may be replaced by a bit line enclosed by a permalloy layer as shown in FIGS. 9 and 11.

Thus, an MRAM device with an improved and novel configuration and its fabrication method are disclosed in which circuitry for controlling magnetic memory element is fabricated first under the CMOS process and then the magnetic memory elements are fabricated. Accordingly, magnetic memory elements are integrated with circuit elements fabricated by a CMOS process and are prevented from degradation of metal composition caused by high temperatures. Further, because digit and bit lines are enclosed by a permalloy layer, magnetic fields generated by digit and bit currents are shielded and focused on magnetic memory elements, and less current is required.